<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        div{
            overflow: hidden; 
            width: 200px;
            height: 200px;
            border: skyblue 1px solid;
            margin: 100px auto;
        }
        div::before{
            content: "";
            display: block;
            width:100%;
            height:100%;
            transform-origin: left bottom;
            transform: rotate(180deg);
            background-color: skyblue;
            transition: all .5s;
        }
        div:hover::before{
            transform: rotate(0deg);
        }
    </style>
</head>
<body>
    <div></div>
</body>
</html>